Mason-Brown, 21, is part of a small team of Brown students to crack a previously undeciphered shorthand used by Williams, the religious thinker and founder of Rhode Island — a mystery that had stumped researchers for years.

The deciphered content of the shorthand, while interesting, isn’t groundbreaking: some of the writing is notes on other essays, while one section outlines Williams’s beliefs on baptizing Native American children. (He was against it.)

“It’s not a Dan Brown novel,” Mason-Brown said, referring to the author of “The Da Vinci Code.” But, he said, it’s “original commentary on several hot-button theological issues of the time.”

Williams, who died in 1683, was a Puritan religious thinker who championed the separation of church and state and advocated for the rights of Native Americans.

“He’s often overlooked, or certainly under-discussed, in the broader context of American history,” Mason-Brown said.

JOHN CARTER BROWN LIBRARY

The margins of a 234-page book in Brown’s library are covered in shorthand written by Roger Williams.

The university’s John Carter Brown Library acquired the book in 1817. Inside, almost every square inch of white space was covered in a cryptic scrawl made up of strange characters dashed off in one or two strokes.

For librarians, the only clue to the origins of the handwriting came from an accompanying note, barely legible itself: The writing, the note said, belonged to Williams.

But how could historians be sure? And, more importantly, what did it say?

That puzzle continued into recent decades.

“There’s this long history of people who know about it, and think of it as this big mystery,” said Linford Fisher, assistant professor in the history department who advised the students. “It’s part of the local lore around Brown and Providence.”

Last fall, a few students approached administrators about taking a stab at cracking the code.

After the Brown Daily Herald published an article about the unsolved shorthand, Mason-Brown wanted to get involved. He is a math fanatic who attended Belmont High School and plans to pursue theoretical math professionally. But something about the shorthand begged for his attention.

“I was sort of instantly captivated,” he said.

Some of the faculty were skeptical: After so long, how could a quartet of undergraduates discern the secret? But others, said Kim Nusco, of the library, were more optimistic.

“We could figure it out,” she recalled a professor saying. “We just need people with brains and time.”

The students had both.

The first step was math, Mason-Brown’s forte. He used a method of statistics called frequency analysis: Guessing that each character stood for a sound or English letter, he found the characters that appeared most frequently in the handwriting. Then, he matched them to the letters that appear most frequently in English — E, T, and A respectively.

“That gives you a foothold to work with,” Mason-Brown said.

Next, the students compared the shorthand with a little-known shorthand dictionary from that era. Williams had based his writing system off standard shorthand from the day, though he changed the characters to suit his needs. Still, there were similarities that helped the students.

It’s a laborious process, Fisher said. By the end of last spring, progress was made, but much of the contents remained hidden. Most of the other students on the team graduated, and Mason-Brown took up the mantle, garnering a grant from the university to spend his summer staring at high-resolution images of the notes on his computer. At this point, he knows all the symbols by heart.

Gretchen Ertl for The Boston Globe

After almost a year of analysis, Mason-Brown says he’s decoded about 75 percent of the writings. None of the notes have anything to do with the essay on which it was written — typical for the period, Williams probably didn’t want to waste paper. And the shorthand wasn’t for secret-keeping — just a method to write his thoughts in a small space. Two large sections of the writings are summaries of essays that had been published during the period.

In another section, Williams rebuts a fellow scholar’s treatise on the baptism of Native Americans. Williams wondered whether it was immoral to force religion on others.

Not exactly fodder for high-octane detective work a la “National Treasure” that Mason-Brown had hoped for, or a startling new discovery.

“My secret hope was that it would turn up new thoughts . . . something a bit more edgy or scandalous,” Fisher said.

But still, Fisher said, the translation is a big deal: It confirms that many of the convictions crystallized earlier in Williams’s life held true until just before his death.

“This is a further elaboration of his views,” said historian John M. Barry, who wrote a book about Williams. “We know that he strongly opposed the use of any kind of pressure or compulsion to convince anyone of any religious belief.”

And perhaps, he said, solving the mystery will bring some much-needed attention to a little-known founding father.

Mason-Brown, who will graduate in May and pursue a Mitchell Scholarship in Ireland, doesn’t plan to pursue cryptography professionally. His ambitions lie more in “pure math” — the secrets of elliptical curves, for example. But he will miss the collaborative nature of his Williams research.

“It’s not often,” he said, “that a math student gets to work on a problem that people in so many different disciplines get to appreciate.”